In lithographic printing, lithographic ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the hydrophilic surface is moistened with water and lithographic ink is applied, the hydrophilic regions retain the water and repel the lithographic ink and the lithographic ink receptive regions accept the lithographic ink and repel the water. The lithographic ink is then transferred to the surface of suitable materials upon which the image is to be reproduced. In some instances, the lithographic ink can be first transferred to an intermediate blanket that in turn is used to transfer the lithographic ink to the surface of the materials upon which the image is to be reproduced.
Lithographic printing plate precursors useful to prepare lithographic (or offset) printing plates typically comprise one or more imageable layers applied over a hydrophilic surface of a substrate (or intermediate layers). The imageable layer(s) can comprise one or more radiation-sensitive components dispersed within a suitable binder. Following imaging, either the exposed regions or the non-exposed regions of the imageable layer(s) are removed by a suitable developer (processing solution), revealing the underlying hydrophilic surface of the substrate. If the exposed regions are removed, the precursor is considered as positive-working. Conversely, if the non-exposed regions are removed, the precursor is considered as negative-working. In each instance, the regions of the imageable layer(s) that remain are lithographic ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water or aqueous solutions (typically a fountain solution), and repel lithographic ink.
“Laser direct imaging” methods (LDI) are used to directly form an offset lithographic printing plate or printing circuit board using digital data from a computer. There have been considerable improvements in this field from use of more efficient lasers, improved imageable compositions and components thereof, and improved processing compositions and procedures.
Various radiation-sensitive compositions are used in negative-working lithographic printing plate precursors as described in numerous publications such as U.S. Pat. No. 7,767,382 (Van Damme et al.) that describes a method of forming lithographic printing plates without rinsing after processing (development). Many other publications provide details about such negative-working radiation-sensitive compositions comprising necessary imaging chemistry dispersed within suitable polymeric binders. After imaging, the negative-working lithographic printing plate precursors are developed (processed) to remove the non-imaged (non-exposed) regions of the imageable layer.
Negative-working lithographic printing plate precursors that can be imaged and processed in known ways can contain polymeric binders and oligomers having acidic groups that promote off-press processing in aqueous alkaline developers (processing solutions). On-press developable imaged precursors depend upon properties of the lithographic printing ink/fountain solution emulsion to develop (remove) non-exposed regions during the initial printing impressions. Irrespective of the mode of development, off-press or on-press, the resulting lithographic printing plates should exhibit good ink receptivity in the imaged (exposed) regions of the printing surface and at the same time exhibit sufficient developability in the non-exposed (non-imaged), especially in the shadows of the printing image.
For on-press developable precursors, the use of polymeric binder in the negative-working imageable layer comprising pendant cyano groups and pendant polyalkylenes oxide segments, as described for example in U.S. Patent Application Publication 2005-0003285 (Hayashi et al.), have been found to be especially useful to provide solvent resistance and higher run length. Such polymeric binders can be present in particulate form.
Hydrophilic co-binders can be included in the imageable layer to enhance processing speed. Oligomers having ethylene oxide groups (with crosslinking sites) can provide desired developability on-press through interaction with the fountain solution. However, such hydrophilic co-binders and oligomers, while providing the necessary speed for on-press development can lead to slower ink receptivity of the image areas. In some cases, the ink receptivity may appear sufficient initially but during the course of the printing process, ink repellency can increase, a phenomena called “blinding”. The result of this problem is that the printed sheets have a weak image without sufficient ink density and therefore the printing job is a failure.
To enable good ink receptivity and reduced “blinding”, negative-working lithographic printing plate precursors can include non-crosslinked hydrophobic binders such as non-crosslinked poly(methyl methacrylate), poly(vinyl acetate), and polystyrene. However, such hydrophobic materials can dissolve in the organic solvents used for coating the imageable layer formulations, and thus forming a film in the imageable layer. Whereas development in non-image (non-exposed) regions can be acceptably fast, slow development in the mid-tone and shadow areas in the printing surface will occur and the screen areas remain clogged throughout a high printing run.
Thus, there are inherent problems from the use of either hydrophilic or hydrophobic co-binders in negative-working lithographic printing plate precursors.
There is a need to further improve the developability in non-exposed regions and ink receptivity in exposed regions in imaged lithographic printing plate precursors that are designed for either on-press or off-press processing.